13-well hexagon pattern for secondary recovery



April 30, 1968 A. F. ALTAMIRA ET AL 3,380,524

Iii-WELL HEXAGON PATTERN FOR SECONDARY RECOVERY Filed June 28. 1966 3 Sheets-Sheet l April 30, 1968 ALTAMIRA ET AL 3,380,524

l3-WELL HEXAGON PATTERN FOR SECONDARY RECOVERY Filed June 28, 1966 3 Sheets-Sheet 2 April 30, 1968 A. F. ALTAMIRA ET AL 3,380,524

13'WELL HEXAGON PATTERN FOR SECONDARY RECOVERY Filed June 28, 1966 3 Sheets-Sheet 5 United States Patent 3,389,524 lit-WELL HEXAGON PATTERN FOR dECGNDARY RECQVERY Anthony F. Altamira and Donald L. Hoyt, Houston, Tex.,

assignors to Texaco lnc., New Yorlr, N.Y., a corporation of Delaware Filed June 28, 1966, Ser. No. 561,108 9 Claims. (Cl. 166-9) This invention relates generally to the production of hydrocarbons from underground hydrocarbon-bearing formations, and more particularly, to a method for increasing the overall production of hydrocarbons therefrom.

in exploiting underground hydrocarbon-bearing formations through a plurality of wells, it has been the general practice that when a production well yields an excessive amount of an extraneous fluid other than the hydrocarbons, e.g., water or gas, that production well is shutin and the production of hydrocarbons is started and carried out at other production wells in the field. It is known that in such instances, a substantial amount of hydrocarbons is left behind in the hydrocarbon-bearing formation since such is not considered primarily recoverable economically.

Secondary recovery operations are now an essential part of the over-all program planning for virtually every oil and gas-condensate reservoir in underground hydrocarbon-bearing formations. Some of the operations developed include gas repressuring and water, fire, steam and solvent flooding. Usually, they all employ some geometric pattern of injection and production wells, with injection of fluid into some of the wells to displace hydrocarbons in the reservoir zone toward the production wells.

The front, or interface, between the injected and inplace fluids moves from injection toward production wells, changing shape as it progresses. Due to the pressure sinks around the production wells, a portion of the interface tends to accelerate and cusp into the production wells. Breakthrough of the injected fluid occurs when the interface reaches the production wells. The percentage of the entire reservoir which has been invaded by the injected fluid at that time is referred to as the sweep efficiency of the particular geometric pattern used.

The most commonly used well pattern in secondary recovery is the 5-spot pattern, with four injection wells at the corners of a square and a production well at the center. In a production held of 5-spot patterns, there are as many injection wells as production wells. Sweep efiiciency for the S-spot pattern is about 71 percent. Other basic flood patterns sometimes used are the 7-spot, direct line and staggered line drives, with sweep efficiencies of 74, 57 and 78 percent respectively.

In field practice, injection usually will be continued well past breakthrough until the reservoir cannot be produced economically. In this way, some additional sweepout can be achieved, but often there will be large volumes of produced injection fluid to be handled, treated and re-injccted. If sweepout prior to breakthrough in a pattern flood were improved, it is very likely that the ultimate recovery would be higher. The time and cost of the operation, to achieve comparable recoveries, would be reduced accordingly.

Therefore, it is an object of the present invention to provide an improved method for the production and recovery of hydrocarbons, particularly liquid petroleum, from underground hydrocarbon-bearing formations.

Another object of this invention is to provide a method whereby the areal sweep efliciency in pattern flooding is improved.

3,380,524 Patented Apr. 30, 1968 ice These and other objects, advantages and features of this invention will become apparent from a consideration of the specification with reference to the figures of the accompanying drawings wherein:

FIG. 1a discloses a conventional S-spot pattern unit in a field undergoing secondary recovery illustrating the interface of the injected fluid at breakthrough at the production wells;

FIG. 112 discloses a conventional 7-spot pattern unit, illustrating the interface at breakthrough of the injected fluid at the production wells in a field undergoing secondary recovery;

FIG. 2 discloses the formation of a 13-well hexagon pattern unit in a field of staggered, equidistantly spaced wells;

FIGS. 3 and 4 illustrate the movements of the interface of the injected fluid during the two phases of the exploitation plan in one 13-well hexagon pattern in a field undergoing secondary recovery; and

FIG. 5 discloses the advance of the interface during the second phase of an alternate exploitation plan, in which the first phase is disclosed in FIG. 3, and this disclosure corresponds to the disclosure in FIG. 4.

in our copending, coassigned application for patent Ser. No. 517,052, filed Dec. 28, 1965, for Interface Ad- Vance Control in Pattern Floods by Use of Control Wells, there is disclosed how an increased amount of hydrocarbons is produced and recovered from an underground hydrocarbon-bearing formation by employing at least three wells, penetrating such a formation, which wells can be in-line, to produce hydrocarbons from the formation via two of these wells including the middle well, as disclosed in the co-assigned US. Patent No. 3,109,487, issued to Donald L. Hoyt, on Nov. 5, 1963.

It is understood that the failure of the driving flood in secondary recovery operations to contact or sweep all the hydrocarbon area is due to the development, in the interface, of a cusp which advances toward the production well. If other portions of the interface could be made to keep up, or if the cusp formation were delayed, complete areal sweep would be possible. In the above cited coassigned application, a production control well is positioned between the injection well and the production well and is kept on production even after the injection fluid reaches it. In this manner, the cusp is pinned down at the control well and while the area swept out by the injection fluid before breakthrough at the last production well is increased, there is an unwanted handling of considerable quantities of injection fluid at the control well.

Another aspect to increase the sweepout efficiency is disclosed in our copending coassigned application for Patent Ser. No. 516,391, filed Dec. 28, 1965, for Interface Advance Control in Pattern Floods by Retarding Cusp Formation, and involves the retardation of the development of the cusp toward the production well. The method of achieving more uniform advance is to control the flow gradients so that the interface is spread out. This can be done either by choosing a particular geometry of well positions or by adjusting the relative production rates so that the velocity of advance is not predominantly in one direction. It can be done also by shifting the gradients frequently, in both direction and magnitude, thus preventing any one section of an interface from advancing too far out of line.

The figures of the drawings schematically disclose and illustrate the practice and the advantages of our invention with well pattern and areal sweepout examples which are obtainable and have been observed both in secondary recovery operations and in potentiometric model studies which simulate secondary recovery operations. The model studies indicate a sweepout obtained in an ideal reservoir,

although the recovery from an actual sweepout of a particular field may be greater or less, depending on field parameters. The procedures to be described are based on the following set of experimental conditions and assumptions:

(1) All the units in any pattern are balanced and produce at the same rate. This requires that wells on the edges of a pattern unit produce or inject at /6 or /2 of the rate of interior wells, depending inversely on the number of pattern units with which they are associated;

(2) The total amount of fluid injected must be equal to the fluid produced for each pattern unit, as well as for the whole pattern;

(3) The mobility ratio of the displacing to the displaced fiuid is 1.0;

(4) The permeability and sand thickness of the formation is uniform; and

(5) Gravitational effects are not considered.

Throughout the drawings, except in FIG. 2, the same symbolic indicators will be maintained as follows: P P and P represents respectively wells at the corners, along the sides, and the interior wells; and, a solid circle indicates a production well, a crossed circle indicates a shutin well, and an arrowed circle indicates an injection well. In FIG. 2, a single circle indicates an original field well, and a double circle indicates an additional well to be drilled in the field.

FIG. 1a illustrates the interface of the injected fluid at breakthrough at the production well of a single conventional 5-s'pot pattern unit in a production field undergoing secondary recovery, wherein the corner wells of each pattern unit are injection wells, while the inner center well is used for production. Such a procedure will produce a sweepout of approximately 71%. In this pattern, the ratio of the distance between the rows of wells to the distance between the in-line wells is 0.5, i.e. d/a= /z.

FIG. 1b illustrates the interface at breakthrough of the injected fluid at the production wells of a unit of an inverted 7-spot pattern unit in a production field, wherein the secondary flooding fluid is injected into the central well and production is maintained at the corner wells until breakthrough, giving a sweepout of about 74%.

In this pattern, the ratio of the distance between the rows of wells to the distance between the in-line wells is 0.866, i.e. d/a=0.866.

Referring to FIG. 2, there is disclosed the formation of a unit of a 13-well hexagon pattern from the conventional 7-spot pattern unit of seven wells. An additional well is drilled in the middle of each side to define the hexagonal pattern comprising six corner wells (P and six wells (P midway between these wells on the sides of the hexagon and a single central well (P,). The wells at the corners or vertices and the central well are part of a staggered equidistant well spacing, the ratio of the distance between the rows of these Wells to the distance between the in-line wells being 0.866, i.e. d/a=0.866. (See FIG. 1b.)

FIGS. 3 and 4 disclose the interface shapes at the ends of a two phase, secondary recovery exploitation plan as used in the 13-well hexagon pattern.

FIG. 3 discloses the interface shape at the end of the first phase of the plan wherein driving fiuid is injected into the formation through the central well with production at the side wells spaced midway between the wells at the corners or the vertices of an equilateral hexagon, until breakthrough of the injected fluid occurs at the side wells. At this breakthrough, a substantially hexamerous area dcfined by the cusp formations ending at the production side wells has been swept, and the area remaining is indicated by cross-hatching in FIG. 3. The ratio of the injection well rate to the production well rate is 3 to 1, i.e. there are three production wells for each injection well throughout the pattern in the field. Both injection and production rates are uniformly distributed.

FIG. 4 discloses the interface shape at the end of the second phase of the exploitation plan, wherein the production side wells (P are converted to injection wells, injection is discontinued through the central well (P which is shut in, and production is initiated at the corner wells (P until breakthrough of the injected fluid thereat, the symbolic indicators being used as noted previously. The sweepout attained at breakthrough of the injected fluid at the corner production wells is about 92% sweep efliciency. The ratio of the injection well rate to the production well rate is 2 to 3, i.e. there are two production wells for each three injection wells, with the production and injection rates being uniformly distributed.

The unswept areas at the end of the second phase as disclosed in FIG. 4 are petal-shaped, clustered around the corner production wells in a region with the strongest flow gradient. This is a significant advantage since in many patterns much of the unswept area is far from the production well, and in the region of such low flow gradient as to be effectively inaccessible.

An alternate second phase of an exploitation plan is disclosed in FIG. 5 and may be used to increase the sweepout efliciency over that for other conventional patterns, e.g., the S-spot, and 7-spot patterns, and corresponds to the disclosure in FIG. 4.

Referring to FIG. 5 which discloses the interface shape at the end of the alternate second phase of this exploitation plan, the side wells are shut in, injection is continued through the central well, and production is initiated at the corner wells until breakthrough of the injected fluid thereat. A total sweepout of approximately 83% is obtained, to average more than 10% greater than that for the S-spot and 7-spot patterns.

As in the case of the preferred exploitation plan as depicted in FIGS. 3 and 4, the unswept areas are located around the production wells, in areas having the strongest flow gradients. Thus, further production may be continued after breakthrough with comparatively less production of the injected fluid than in the ordinary production following breakthrough.

FIGS. 4 and 5 exemplify the interfaces obtained by spreading the cusp as disclosed in the latter cited copending application.

It is recognized that any of the increased sweepouts obtained by the use of a 13-well hexagon pattern, exploited as disclosed herein, is an idealization, as in the case for the sweepouts for the 5- and 7-spot patterns. None of the values is likely to be achieved in the field, but the relative superiority of the sweepouts obtained by the use of the 13-well hexagon pattern is clear and is independent of inhomogeneities.

The basic technique employed to increase sweep efficiency controls the advance of the front in the pattern to achieve large areal coverage, by shifting and adjusting the geometric position, direction and magnitude of the flow of pressure gradients. If the positions of the wells in the pattern are favorable, by changing the wells used for injection and produc-tion, high flow gradients can be made to cover most of the pattern. The patterns and procedures disclosed herein have high sweep efficiency leaving the unswept areas at breakthrough in the regions of strong flow gradients adjacent the production wells, and thus readily swept by production past breakthrough.

The sweepout and volumes of injected fluid produced vary from one pattern to another, and also appear to be functions of rate distribution and distance parameters within a given pattern.

In evaluating the performance of a flood pattern, or in comparing performances of different patterns, there are three main considerations:

(a) Percentage of sweep (b) Volume of injection fluid handled (0) Time to achieve the sweep For a given total field production rate, it will not be possible generally to obtain an increase in swecpout without at least a proportionate increase of time, which is to be expected. However, if the extra time involved is disproportionately long, the gain in sweepout may not be economically worthwhile.

Any pattern and/ or rate distribution which retards the development, or the advance, of a cusp towards the production wells will increase the sweepout of a field. As mentioned previously, two principal means of doing this are: (a) pinning down the cusp by locating production Wells between the injection source and the outer production wells, and keeping these inner (or control) wells on production after breakthrough, and ('b) spreading out the cusp by pulling the front toward side wells until breakthrough thereat before allowing it to proceed toward the corner production wells of a pattern unit, as disclosed in our above cited coassigned applications.

The spreading out of the cusp is in general a more advantageous procedure. It yields as good or better sweepout with less production of injection fluid. Further, a higher rate distribution on corner wells of pattern units generally results in much less overall production of injection fluid, but also in less sweep (although exceptions can be found in more complicated patterns).

The advantages of the methods disclosed above are evident. Fewer injection wells are required, more reservoir fluid is recovered prior to breakthrough of injection fluid, and so more ultimate recovery is obtained, as compared with other methods generally employed in secondary recovery operation.

Although emphasis has been placed in this disclosure on the practice of this invention as directed to a secondary recovery operation, particularly employing water or other similar aqueous fluid as the injection fluid or displacement fluid, the advantages obtainable in the practice of this invention are also realized in primary hydrocarbon production operations wherein the hydrocarbon-bearing formation is under the influence of a water drive or gas drive, or both a water and a gas drive, and also in the instance of a secondary recovery operation wherein a gas, such as natural gas, is employed as the injection fluid.

As will be apparent to those skilled in the art, in the light of the accompanying disclosure, other changes and alternatives are possible in the practice of this invention without departing from the spirit or scope thereof.

We claim:

1. A method of producing hydrocarbons from an underground hydrocarbon-bearing formation involving wells located at the vertices and on the sides of a hexagon and a well located therewithin which comprises introducing fluid into said formation via the well located Within said hexagon, producing hydrocarbons from said formation via the wells located on said sides of said hexagon until breakthrough thereat of said fluid introduced into said formation, and thereupon respectively ceasing introducing fluid and producing hydrocarbons via said wells within said hexagon and on said sides, and commencing introducing fluid into said formation via the side wells and initiating producing hydrocarbons at said wells located at said vertices of hexagon.

2. In the method of producing hydrocarbons as defined in claim 1, the rates of introducing fluid into and producing hydrocarbons from said formation being controlled so that breakthrough of the introduced fluid at said production wells occurs simultaneously.

3. In the method of producing hydrocarbons as defined in claim 1 said wells located at said vertices of said hexagon defining an equilateral structure, and said well located therewithin being positioned equidistantly from said vertices.

4. In the method of producing hydrocarbons as defined in claim 3, said wells located on said sides of said hexagon being spaced equidistantl from said vertices therealong whereby said wells at said vertices and on said sides are spaced equidistantly from each other on each of said sides.

'5. In the method of producing hydrocarbons as defined in claim 4-, said wells being located equally at said vertices of and on said sides of and within said hexagon and being at least thirteen in number, defining a unit in a production field.

6. The method of producing hydrocarbons from an underground hydrocarbon-bearing formation involving wells located at the vertices and on the sides of a hexagon and a well located therewithin which comprises injecting fluid into said formation through said well located within said hexagon and producing formation hydrocarbons from the wells situated on said sides of said hexagon until breakthrough of the injected fluid thereat, and thereafter continuing injecting fluid into said formation through said well within said hexagon and ceasing producing formation hydrocarbons from said side wells, and initiating producing formation hydrocarbons from the wells at said vertices until breakthrough of injected fluid thereat.

7. In the method of producing hydrocarbons as defined in claim 6, the wells at the vertices and on the sides of said hexagon being spaced equally from each other along the periphery thereof, the well within said hexagon being located equidistantly from said vertices of said hexagon.

'8. In the method of producing hydrocarbons as defined in claim 6, the wells on said sides of said hexagon being located intermediate the wells located at said vertices of said hexagon, and the well within said hexagon being located intermediate said wells at the vertices thereof, the wells defining and within said hexagon being at least thirteen in number, as part of a unit in a production field.

9. In the method of producing hydrocarbons as defined in claim 7, the wells located at said vertices of and within said hexagon being spaced apart and aligned in rows so that the ratio of the distance between the rows of wells to the distance between the in-line wells is 0.866.

References Cited UNITED STATES PATENTS 2,885,002 5/1959 Jenks 166-9 3,113,616 12/1963 Dew et al. 166-9 3,113,617 12/1963 Oakes 166-9 3,113,618 12/1963 Oakes 166-9 3,120,870 '2/ 1964 Santourian 166-9 3,143,169 8/1964 Foulks 166-9 3,205,943 9/1965 Foulks 166-9 CHARLES E. OOONNELL, Primary Examiner. D. H. BROWN, Examiner. 

1. A METHOD OF PRODUCING HYDROCARBONS FROM AN UNDERGROUND HYDROCARBON-BEARING FORMATION INVOLVING WELLS LOCATED AT THE VERTICES AND ON THE SIDES OF A HEXAGON AND A WELL LOCATED THEREWITHIN WHICH COMPRISES INTRODUCING FLUID INTO SAID FORMATION VIA THE WELL LOCATED WITHIN SAID HEXAGON, PRODUCING HYDROCARFBONS FROM SAID FORMATION VIA THE WELLS LOCATED ON SAID SIDES OF SAID HEXAGON UNTIL BREAKTHROUGH THREAT OF SAID FLUID INTRO- 